1. Field of the Invention
The present invention generally relates to machining equipment and processes. More particularly, this invention relates to a method and apparatus that combines a fluid-jet system and an electrical-discharge machining (EDM) system for use in the repair of air-cooled airfoil components of gas turbine engines.
2. Description of the Related Art
Components located in the high temperature sections of gas turbine engines are typically formed of superalloys. Such components, which include combustors and turbine nozzles (vanes) and buckets (blades), are under strenuous high temperature conditions during engine operation, which can lead to various types of damage or deterioration. For example, erosion, cracks and other surface discontinuities tend to develop at the trailing edges of airfoils (e.g., buckets and nozzles) during service due to foreign object impact (foreign object damage, or FOD).
Because the material and processing costs of superalloys are relatively high, repair of damaged or worn superalloy components is typically preferred over replacement. For this purpose, weld repair methods have been developed using tungsten inert gas (TIG) and other welding processes.
The first and second stage power nozzles of industrial gas turbine engines are notably prone to damage caused by impact with foreign objects. For purposes of discussion, a section of a nozzle segment 50 is represented in FIG. 1, in which multiple nozzle partitions 52 are supported between a pair of bands 54. In a typical repair process, the nozzle segment 50 is removed and then undergoes repair by hand. The damaged area of the nozzle segment 50 may be a small surface region of the segment 50, such as the trailing edge 58 of a partition 52, or encompass a much larger area. If the former, the damaged area can be selectively removed by grinding using a high speed grinder with a burr attachment, while the latter may require removal of an entire partition 52 using a high speed grinder with an abrasive cutting disc. Each of these operations is labor-intensive, often requiring about four man-hours or more. After removal of the damaged area, the repair process is completed by welding and grinding. If a partial partition 52 has been removed, a replacement may be welded in its place. Smaller surface areas are repaired by TIG welding to build up a weldment that replaces the removed material. The welding process is followed by grinding in order to closely duplicate the original contours (e.g., suction and pressure surfaces) of the partition 52.
Weld repairs of air-cooled turbine components, such as the partitions 52 of FIG. 1, are further complicated by the presence of cooling holes 60, which are typically formed at the trailing edge 58 by such drilling techniques as electrical-discharge machining (EDM) and laser machining. During welding, cooling holes 60 in the surfaces of a nozzle partition 52 are susceptible to blockage by weld filler material that enters the holes 60. The performance of a partition is directly related to the ability to provide uniform cooling of its surfaces with a limited amount of cooling air. In particular, the size and shape of each hole 60 determine the amount of air flow exiting the hole 60 and the distribution of the air flow across the downstream surface of the partition 52, and also affect the overall flow distribution within the cooling circuit containing the hole 60. Consequently, it is important that the cooling holes 60 in a weld-repaired partition are substantially restored to their original size, shape and location. Methods for reestablishing cooling holes or blocking existing cooling holes are known, such as through the use of carbon rods. However, this technique challenges the welder in retaining the integrity of the weld around the carbon rod, and often requires rework.
In view of the above, it can be seen that the removal and repair of a gas turbine airfoil component is labor-intensive, particularly with the added demand that the contours and cooling holes of the repair closely duplicate that of the original component. While various other approaches have been proposed for repairing nozzle partitions, such as in commonly-assigned U.S. Pat. No. 5,895,205 to Werner et al., there is an ongoing effort to develop improved repair methods.
The present invention provides a method and apparatus for repairing an article, and particularly an air-cooled airfoil, during which at least a portion of the airfoil must be removed and replaced. The method and apparatus make use of a combined fluid-jet system and an electrical-discharge machining (EDM) system that enables the contours and cooling holes of a repaired airfoil to closely duplicate that of the original.
The apparatus of this invention includes at least one workpiece holder adapted to position and secure an airfoil on the apparatus, a multi-axis head adapted for movement relative to an airfoil positioned and secured on the apparatus, a nozzle mounted to the multi-axis head and operable to remove at least a portion of the airfoil with a jet of fluid discharged therefrom, an electrical-discharge electrode mounted to the multi-axis head and operable to form surface holes in the airfoil by electrical-discharge machining, and means for controlling the movement of the multi-axis head. More particularly, the controlling means is operable to precisely position and move the nozzle relative to surface contours of the airfoil when removing the portion of the airfoil, and to precisely position and move the electrical-discharge electrode relative to surface contours of the airfoil when forming the surface holes in the airfoil.
The above-described apparatus makes possible a method of repairing an air-cooled airfoil by positioning the airfoil on the apparatus, operating the multi-axis head to remove at least a portion of the airfoil by cutting the airfoil with a jet of fluid discharged from the nozzle mounted to the multi-axis head, removing the airfoil from the apparatus, welding the airfoil to form a replacement section that replaces the portion removed from the airfoil, positioning and securing the airfoil to the apparatus with a workpiece holder, and then operating the multi-axis head to form surface holes in the replacement section of the airfoil by electrical-discharge machining the replacement section with an electrical-discharge electrode mounted to the multi-axis head.
In view of the above, the apparatus and method of the present invention are able to improve the productivity, quality and safety of the operation of repairing an air-cooled airfoil by combining equipment for two separate cutting operations on a single multi-axis head that is configured and controlled to be highly and precisely maneuverable. Use of a multi-axis head enables movement of both the fluid-jet nozzle and the electrical-discharge electrode to be controlled so as to precisely position and move the nozzle relative to surface contours of the airfoil when removing the portion of the airfoil, and later to precisely position and move the electrical-discharge electrode relative to surface contours of the airfoil when forming the surface holes in the repaired airfoil, based on contour data that can be stored by the apparatus.
Other objects and advantages of this invention will be better appreciated from the following detailed description.